Fixed UnitOfWork::recomputeSingleEntityChangeSet exception with STATE_REMOVED entities

Hi!

I found a bug in UnitOfWork::recomputeSingleEntityChangeSet for the branch 2.4. I didn't tested master and previous version, but the bug may also apply.

The problem is that an exception is thrown on flush (at least when using a single entity flush), if an entity is in deleted state and also with pending modification and a preUpdate listener configured.

I've attached a patch and a unit tests (I've tweeked a previous preUpdate method used for testing, but everythings pass).

Anyway, this result in an UPDATE followed by a DELETE statement. I think also that an entity in STATE_REMOVED don't need to be updated, and should be immediately deleted. If you agree, I could implement it myself (as long I manage not to break unit tests).